1. Field of the Invention
The present invention relates to a projection-type multi-color liquid crystal display device.
2. Description of the Related Art
A projection-type display using a liquid crystal, in which a display is performed by a transmission and scattering of light, is distinguishable over the TN (twisted nematic) type liquid crystal display or the ferroelectric liquid crystal display in that a bright display that can be easily seen is obtained because a polarizing plate can be omitted. As the liquid crystal display method of this type, there are known a laser heat writing method using a smectic A phase liquid crystal and a method using a phase transition type liquid crystal as previously proposed by the present inventors. When a phase transition type liquid crystal is used, since the voltage hysteresis in the phase transition between the cholesteric phase and the nematic phase is utilized, a large-information display is possible with a simple matrix structure ("New Nematic-Cholesteric LCD Using Hysteresis Behavior", SID 1985; U.S. patent application Ser. No. 06/832,897, filed on Feb. 26, 1986).
The principle of a display using a phase transition type liquid crystal will now be described in detail with reference to FIG. 1. In FIG. 1, the applied voltage is plotted on the abscissa and the non-scattering transmission, that is, the ratio of the quantity of light transmitted through the liquid crystal phase without scattering, is plotted on the ordinate. When the voltage is low, the liquid crystal is in the cholesteric phase (focalconic texture and light is scattered outwardly of a projecting image-forming lens, and therefore, a dark zone is formed on the screen. When the voltage is high, the liquid crystal is in the nematic phase (homeotropic texture) and light is incident on the image-forming lens without scattering, and a bright zone is formed on the screen. Since the phase transition voltage upon elevation of the voltage is different from the phase transition voltage upon a drop in the voltage, a certain kind of hysteresis occurs. This hysteresis is utilized in the above-mentioned phase transition type liquid crystal. Namely, an alternating current voltage of 2 Vd is first applied to render the entire liquid crystal transparent. In this state, the voltage is then reduced to Vd to write transparent image elements, and the phase of the voltage applied to upper and lower electrodes is deviated by one pulse and reduced to 0 of the field intensity to write opacity image elements. Then, a voltage of Vd is held to keep the display. In the conventional direct-vision TN type liquid crystal, a color display method is known in which RGB (red-green-blue) filters are used and a displayed image is provided by light from a background light source. Also in the projection-type display system using a phase transition type liquid crystal, colorization is effected according to the same principle. However, if RGB filters are used, the quantity of projected light is reduced to less than 1/3 and a projected image is dimmed by external light. Therefore, only a display having a low contrast is obtained. Moreover, the resolution is also reduced by 1/3.
It is possible to perform a color display by mixing a plurality of light fluxes, which have passed through a plurality of liquid crystal panels, by using a dichromatic mirror. In this case, however, the size of the apparatus is increased and the apparatus becomes expensive. Accordingly, a projection-type color display has not been practically utilized.
Color display devices using a cholesteric-nematic phase transition type liquid crystal are known, although these are direct vision type devices, not projection type (see Japanese Unexamined Patent Publication (Kokai) Nos. 59-116614, 59-116680, 52-133781, 61-32801, 60-29091, 57-201218, 59-18925, 57-124712 and 57-111514 and F. J. Kahn "ELECTRIC-FIELD-INDUCED COLOR CHANGES AND PITCH DILATION IN CHOLESTERIC LIQUID CRYSTALS"(PHYSICAL REVIEW LETTERS Vol. 24, No. 5, Feb. 2, 1970, pp 209-212)). Moreover, the principles of coloring for these color display devices are different from that in a multi-color display device according to the present invention.